What if every D&D power could be used at will?

Seriously, what would happen?

I think that in D&D 5e:

• Resource tracking is fiddly, takes valuably game time, and isn't particularly fun.
  
• The game's built-in resource management is one of its weakest design elements. So much hinges on the long rest mechanic that resources feel all-or-nothing. Between published adventure content and the relative lack of 'living world' rules under the hood, resting tends to be either impossible or unpunished, with little middle ground, depending on the situation. The DM gets little guidance on making it work.
• Resource management decisions are only sometimes important, and seldom very engaging or fun, compared to the other decisions made during play. You learn good spells and tactics, and it's seldom important to change them. You may sometimes have intel leading to an opportunity to prepare differently, but the game isn't so difficult that doing so is necessary, and there isn't much in the way of DM guidance or built-in mechanics for getting intel into the PCs' hands.
• Most of the game's rules are about tactical life-or-death combat, and therefore the game itself is about tactical life-or-death combat (although of course not everyone plays it this way). Trying to conserve resources in combat is a big gamble. After the particularly resource-limited early levels, play styles tend to converge on 'open with full broadside' and not be punished for that.
• It is a game where characters don't just have 'powers' but extravagant superpowers, and it's highly character-focused.
• Character classes all have a fairly similarly design, considered in the abstract. They get an array of at-will small superpowers plus various rate-limited superpowers, almost all of which can be used a low-single-digit number of times between rests. Other limiters on powers (e.g. HP or GP costs) are rare.
• The game's balance is precarious and a DM needs to develop skill to tread the line between 'feeling dangerous' and 'being unfair'.

Let's say you mostly agree with these premises. I think it follows that making all D&D 5e powers at-will – that is, removing their rate limits...

• Would remove part of the game, but not an important one
  
• Wouldn't get rid of interesting choices
• Would work for all classes without changing typical playstyles
• Would lean into the existing superpowered and character-focused feeling of the game
• Would make gameplay much faster
• Wouldn't upset a perfect game balance that doesn't exist.

Spam spells all you like. Action surge on every turn. Put ki points on every attack; metamagic on every spell; enter rage or confer bardic inspiration whenever you can spare a bonus action.

This is an idle fancy – I've never seen it done. But I'd be fascinated to see how it played out. Extrapolating some consequences that seem plausible:

1. The DM could then abandon CR and offer challenges which would otherwise be completely unfair, then see what the players come up with.
2. If the DM created a 'too easy' encounter as a result, it wouldn't matter, because the power disparity and lack of resource tracking means the encounter gets settled in a fraction of the time that combat, spell-based trickery, or the various resource-conserving hedges usually take at the table.
3. The DM could alternatively keep CR but remove all the rate limiters on adversaries' resources: recharges, dailies, slots, etc. (But unlike character classes, this would affect some creatures much more than others)
4. PCs wouldn't need to keep lesser powers and spell choices for backup offense. With normal ideas of challenge abandoned there should be more situations where a particular utility spell or defence is desperately needed. That suggests that spell selection might become a more interesting decision!

Now, historically this sort of thing would lead to a worsening of 'linear fighters, quadratic wizards', but I don't think the difference would be as large in this edition of the game, at least before the highest levels. Fighters getting two actions every turn via action surge is pretty huge, for example. The biggest difference might be in high-level utility powers, the kind where you normally have one use of a valuable high-level spell slot and have to consider whether to save it for a fight. Now you could Fabricate or create a permanent Wall of stone every ten minutes, Mind blank the whole party, spam divination spells, and so on.

What other issues would you need to navigate?

• A few character powers stop mattering. For example, second-rate but 'cheap' healing features are mostly obviated by having lay on hands or a decent healing spell. You might need to introduce some 'alternative features' available to all classes.
• Some character powers would become irrelevant as the character increased in level. This wouldn't matter except that I doubt they're evenly distributed over character classes. You'd want to add a mechanic for swapping defunct powers out once you grow out of them, for those classes that don't already have one.
• There are some slightly more or less resource-limited classes, and they become slightly better or worse choices respectively as a result. I don't think this matters except for warlocks, who deviate from the overarching class design patterns. They'd keep their major disadvantage (few powers) while everyone gets a better version of their major advantage (powers usable more often).
• At the highest levels, powers (and spells especially) can become absurdly strong. But having poked around, I think only a few of them would be real trouble. It might literally just be Wish and Divine Intervention!
• If you're abandoning material costs from spells as well as spell slots, you'd want to look at the few spells that are highly constrained by GP cost: Forcecage, Clone, Sequester, Gate, etc.
• I might be wrong; maybe top-tier spellcasting really does outcompete other classes. You could make level 8-9 spells the sole exception to the 'infinite resources' rule, perhaps sticking to slots or limiting them in some other way (once per hour?).
• Rogues would now get infinite Stroke of Luck as their capstone: their attacks always hit and their ability checks always succeed. I don't think this actually outshines having infinite spells, but it might be boring.
• If the PCs' opponents also have no constraints, there might be a risk at high levels of the game devolving into win-initiative-to-win-combat, but hopefully it would lead to unique solutions.

Implications for the world

Obviously things would get absurd, especially if this is the norm for all people in the setting. But I already think the implied and official D&D settings don't actually develop most of the implications of the things that are canonically possible within them. All sorts of things (technologies, prices, lifestyles, community structures, social structures, etc) ought to look completely alien on the basis of low-level spells alone, and yet the setting (with its dramatic mixture of tech levels, dramatic mixture of society types, dramatic mixture of stakes, etc) stands unaffected.

But overall, losing even more immersion in a typical fantasy setting does strike me as a glaring issue, yes. Maybe abandoning rate limits would work in a more (post-)apocalyptic setting? One where everything is chaotic and in flux, where huge changes happen in the world very quickly, where absurdly spectacular powers are harnessed to mundane ends, and where nightmarishly powerful entities regularly make an appearance?